Typical autofocus cameras recently available in the market are so arranged that the light from an object to be photographed falls upon an image sensor and focus detection is performed on the basis of an image data signal (i.e., image pickup signal) output from the image sensor.
Many autofocus cameras use a charge-coupled-device (CCD) element or metal-oxide-semiconductor (MOS) sensor as the image sensor and, in general, employ a so-called "phase difference detecting mode" in which the object to be photographed is imaged both on a primary area and a reference area of the image sensor. Then the image on the reference area is brought into proper correlation with the image on the primary area to effect the desired focus detection. Other autofocus cameras employ a so-called contrast detecting mode which utilizes the fact that the object imaged on the image sensor exhibits the highest contrast ratio when brought into focus.
FIG. 17 is a block diagram illustrating an example of a prior art image sensor. It consists of a CCD element which is well known, comprising a multi-pixel image sensor array 11 having a primary image area and a reference image area, a reset gate 12, a shift gate 13, and a CCD shift register 14. A brightness monitoring sensor 15 disposed adjacent the image sensor causes its associated amplifier 16 to output a brightness monitoring voltage V.sub.m which is used to regulate the charge accumulation or storage time of image sensor array 11.
More specifically, as shown in FIGS. 18A and 18B and FIGS. 19A-19C, in response to a reset pulse .phi..sub.r input to the reset gate 12 of sensor array 11 and the circuit associated with monitoring sensor 15, an amplifier 16 begins-outputting a falling brightness monitoring voltage V.sub.m, the slope of which is determined by the brightness of the light incident on monitoring sensor 15. The pixels of image sensor array 11 accumulate negative charge in response to the image illumination intensity for a charge accumulation time interval which ends when brightness monitoring voltage V.sub.m falls to a preset voltage level V.sub.t.
In response to a shift pulse .phi..sub.s, the amount of charge thus stored in array 11 is transferred in parallel to the CCD shift register 14. Then, in response to transfer clock pulses .phi..sub.1, .phi..sub.2, the charges stored in the shift register 14 are successively serially transferred to a preamplifier 17 which, in turn, outputs a corresponding image data signal voltage V.sub.os.
This image data signal voltage V.sub.os is compared to a compensation voltage V.sub.cs output from a reference amplifier 18 identical to amplifier 17, and a signal (V.sub.os -V.sub.cs) representing the difference of these voltages is digitalized by an analog-to-digital (A/D) converter (not shown) and then applied to a digital signal processing circuit (not shown) for distance calculations.
In this arrangement of the prior art, the monitoring sensor 15 monitors only an average brightness of the illumination distributed on the primary area of the image sensor array 11. Accordingly, as shown in FIG. 20, when an image of a high contrast ratio is projected onto the sensor array 11, or relatively intense light is incident upon some pixels locally, the brightest pixels become saturated as time increases as is shown happening in charge amount curve A. Because the saturated pixels distort curve A, an accurate image data signal voltage V.sub.os cannot be obtained, and an error inevitably appears in the resulting distance calculations used for focusing.
To avoid such saturation with high contrast images, it might be contemplated to set the preset voltage level V.sub.t in FIGS. 18A and 18B for the monitoring voltage V.sub.m to a level corresponding to a shorter charge accumulation time for image sensor array 11. However, this would have the disadvantage of decreasing the amount of charge accumulation when detecting an object of a low contrast ratio and reducing the signal-to-noise (S/N) ratio of the image data signal voltage V.sub.os. Moreover, a level of V.sub.t sufficiently far from the saturation region would limit the amount of stored charge, causing the image sensor to be inefficiently used at a disadvantageously low S/N ratio.
Furthermore, the prior art brightness monitoring sensor 15 is never exposed to exactly the same light from the object to be photographed, or a portion thereof, as the light incident on the image sensor itself. Therefore, it is sometimes impossible to properly control the amount of accumulated charge, depending upon the particular objects to be photographed.
As shown FIG. 18B, when the image of an object projected onto the sensor array 11 is of a low contrast ratio, the signal fluctuation-(V.sub.max -V.sub.min) of the image data signal voltage V.sub.os is substantially smaller than the fluctuation in the difference voltage-(V.sub.os -V.sub.cs) between the image data signal voltage V.sub.os and the compensation voltage V.sub.cs. Therefore, if the signal to be digitalized for data processing is (V.sub.os -V.sub.cs), and A/D converter of relatively high resolution must be employed to accurately convert the information-bearing ripples in image signal V.sub.os.
As will be apparent from FIGS. 18A and 18B, and FIGS. 19A-19C, the charge accumulation time of the image sensor array 11 is set to the time elapsing from when the reset pulse .phi..sub.r is input to the time at which the monitoring voltage V.sub.m (corresponding to an average output signal) reaches the preset voltage level V.sub.t. Thus, the average of the difference voltage-(V.sub.os -V.sub.cs) is kept constant and such difference voltage is input to the A/D converter after amplification.
In the special case when the brightness of the object to be photographed is so low that the brightness monitoring voltage V.sub.m does not reach the preset voltage V.sub.t within a specified time limit, the difference signal voltage-(V.sub.os -V.sub.cs) for the image is amplified by an amount determined by the level reached by the average brightness monitoring voltage V.sub.m. In such case also, no effective A/D conversion can be achieved for an object of low contrast ratio, since the degree of amplification depends on the average of said difference voltage-(V.sub.os -V.sub.cs) rather than the range of image signal fluctuation V.sub.min -V.sub.max.